In recent years increasing attention has been given to the importance of protecting the eyes and skin from radiation emitted by artificial and natural light sources. Prolonged exposure of the eyes to reflected solar ultraviolet radiation is believed to result, for example, in the formation of cataracts and general tissue damage in the retina. Furthermore, attention has been given to the importance of protecting packaged material from such radiation to reduce the destabilization, degradation, decay or other undesirable effects on that material that may be caused by the radiation.
A variety of commercial optical filters have evolved to meet the threats posed by these radiation environments. Such optical absorption systems include sunglasses, contact lenses, aircraft and automobile windows, welders glasses and others.
In the case of sunglasses, two general types of materials are currently employed as practical absorbing components. Metallic films deposited onto plastic lens substrates are very effective sunscreens providing broad band attenuation of electromagnetic waves from the ultraviolet into the near infrared region of wavelengths. However two disadvantages are associated with this type of sunglass system. Manufacturing steps beyond the formation of the basic plastic lens are required and secondly, waves incident from the rear and reflected directly into the eye pose a new problem and require further manufacturing modifications.
Dyes and pigments comprise the second general class of optical absorbers. These molecular or polymeric elements are either deposited as thin films or are dispersed into the plastic matrix. U.S. Pat. No. 4,157,892 illustrates a method of coloring water-absorbable plastics. Disadvantages of this type of system are often the inability of the dye or pigment to absorb radiation sufficiently over all the ultraviolet wavelengths and a tendency to photodegrade. Photodegradation is particularly common to organic dyes and pigments.
Prior art does exist for melanin as a sunscreen; however, this prior art is restricted to the use of melanin as an ultraviolet protecting pigment in a cosmetic cream applied to the skin (see Japanese Patent-Kokai-74 71, 149).
The use of melanin, an easily synthesized biopolymer, as a sunglass pigment, offers several advantages over the prior art. These advantages will become evident in the following description.
For the purpose of the present description, melanins are defined and classified as in the book entitled Melanins, by R. A. Nicolaus, published in 1968 by Hermann, 115, Boulevard Saint-Germain, Paris, France (hereinafter "Nicolaus") which work in its entirety is incorporated herein by reference. As defined by Nicolaus, melanins constitute a class of pigments which are widespread in the animal and vegetable kingdoms. Melanins are macromolecules consisting of mixtures of polymers that are highly conjugated in nature. The extensive degree of conjugation produces their unique transmittance spectrum. Melanins are highly irregular, three-dimensional polymers not only in the way monomeric units are linked together but in the nature of the units themselves. A typical melanin structure is shown in Arnaud, R., et al, Photochem Photobiol, Vol 38, page 161-168 (1983), Electron Spin Resonance of Melanin from Hair, Effects of Temperature, pH, and Light Irradiation. While the name melanin in Greek means black, not all melanins as pigments are black but may vary from brown to yellow. Melanins are classified in three groups, namely, eumelanins, phaeomelanins and allomelanins. Eumelanins are derived from the precursor tyrosine shown as compound (1): ##STR1## Phaeomelanins are derived from the precursors tyrosine or cysteine shown as compound (2): ##STR2##
Allomelanins, the meaning of which is other melanins, are formed from nitrogen- free precursors, such as catechol. It is also believed that 1,8-dihydroxynapthalene may produce melanin through enzymatic oxidation. Further information on Melanins is found and incorporated herein by reference on page 827, Monograph No. 5629 in The Merck Index (10th Ed.1983). Melanin is produced in nature by the oxidative polymerization of the precursors. Furthermore, melanin may be synthesized commercially or in the laboratory from precursors through the free radical polymerization of these precursors. This invention is directed to the use of any melanin regardless of its source or method of preparation. Therefore, natural, synthetically prepared or any other melanin may be used in accordance with the present invention as an absorbing pigment. An example of the synthetically produced catechol melanin and DOPA melanin are found in the article by Froncisz, W., Sarna, T., Hyde, James S. Arch. Biochem. Biophys. "Copper (2+) ion Probe of Metal -ion Binding Sites in Melanin Using Electron Paramagnetic Resonance Spectroscopy." I. Synthetic Melanins. (1980, 202(1), 289-303). That article is incorporated herein by reference. The catechol melanin is disclosed in the Froncisz et al. article as being produced as follows:
Catechol Melanin. A solution of 15 g of catechol in 3 L of deionized water was brought to pH 8 with ammonium hydroxide, and then air was bubbled through the stirred solution for four days. The resulting melanin was precipitated by addition of concentrated hydrochloric acid to bring the pH to 2, then washed with dilute HCl and dialyzed against deionized water for several days to remove H.sup.+ and Cl.sup.- ions. The concentration of the melanin suspension was estimated by drying an aliquot in vacuum over phosphorus pentoxide and weighing. Oxidized catechol melanin was prepared by adding 10 mL of 10.sup.-3 M potassium ferricyanide to 30 mg of melanin and incubating for 10 minutes. The suspension was then spun down, washed twice with deionized water and suspended in 5 mL of deionized water.
Examples of enzymatically produced melanin are found, among others, in the following articles:
Blois, Zahlan and Maling, Electron Spin Resonance Studies on Melanin, Biophys. J. (1964, 4, 471);
Woert, Prasad and Borg, J. Neurochem. (1967, 13, 707);
Chauffee, Windle and Friedman, Electron Spin Resonance Study of Melanin Treated with Reducing Agents, Biophys. J. (1975, 15, 563-571);
Binns, Chapman, Robson, Swan and Waggott, Studies Related to the Chemistry of Melanins. Part VIII. The Pyrrol-carboxylic acids formed by oxidation or Hydrolysis of Melanins Derived from 3,4 - dihydroxyphenethylamine or (.+-.) -3,4 dihydroxyphenylalanine, J. Chem. Soc.(c) (1970, 1128-1133).
These articles are incorporated herein by reference. A typical enzymatic preparation of melanin is disclosed in the Chauffee et. al. article as follows:
A solution of 30 milligrams of purified mushroom tyrosinase (monophenol monooxygenase) in 100 milliliters of Sorensen's buffer of pH of 7 was added to 150 milligrams of L-Dopa (3,4 -dihydroxy phenylalanine) in 500 milliliters of the same buffer solution or to 500 milliliters of a buffer solution saturated with tyrosine. After reaction for two weeks, the precipitated black pigment was filtered, washed with water, and dried over phosphorus pentoxide.